In a vehicle equipped with an electric power steering (EPS) system, a steering assist force is provided by an electric motor operatively coupled to a steering wheel through a steering column or shaft. Typically, such systems include a controller programmed to derive an assist torque signal along with a “return to center” torque signal, thereafter summing these torque signals to generate a motor command signal that is applied to the electric motor. The assist torque signal is derived using an applied torque signal indicative of torque applied to the steering wheel by a vehicle operator. The assist torque signal provides a power steering boost torque, and the “return to center” torque signal provides a bias torque for returning the steering wheel to a system zero position. The system zero position may be defined as a position for which the vehicle will travel along a linear, substantially straight-ahead path.
During EPS operation, it may be desirable to automatically determine the absolute position of the steering wheel. This position may be used, for example, to cause the steering wheel to return to center following a steering input in the form of torque applied to the steering wheel by a vehicle operator. The return to center effect simulates the self-aligning torque due to positive caster of a conventional hydraulically-operated steering system, and causes the vehicle to be more comfortably controlled by a driver.
In order to automatically determine the center position of the steering wheel in an EPS system, a motor position sensor may be employed to measure a relative angular position of the motor. However, since the motor position sensor senses relative position, the EPS system utilizes a second sensor to determine an absolute reference for the center position of the steering wheel. This second sensor is provided in the form of a steering wheel sensor mechanically connected to the steering wheel, steering column, or shaft for producing a sensor output signal indicative of the angular position of the steering wheel. Since the steering wheel may be capable of rotating through three or more revolutions, the sensor output signal may exhibit periodic errors. These periodic errors may recur at substantially the same angular position of the steering wheel for each of a plurality of rotational cycles.
Errors in the steering wheel sensor output signal are partially attributable to the mechanical connection of the steering wheel sensor to the steering wheel, steering column, or shaft. Errors are also attributable to the physical properties of the sensing mechanism used to generate the sensor signal. But regardless of the source of this error, if the steering wheel sensor output signal is used to establish the center position of the steering wheel, either the sensor must provide adequate accuracy, or an appropriate error compensation scheme must be devised. Since improvements to sensor accuracy require increasing the precision of components, materials, and mechanical tolerances, such an approach is costly and impractical. What is needed is an efficient compensation technique for correcting errors in the steering wheel sensor signal.